pupal behaviour

I am a high school senior. In biology II we have a project. We are raising
drosophilas to study the genetic variations. All what happened in my flask was
pretty normal ( similar to my classmates ). But now most of my pupas stayed on top
of the food. My colleagues' pupas are up the walls. I am worried because I suspect
that my experiment is going wrong. Any body have seen this behaviour before? Are
my pupas dead? I will appreciate your answers and advices. Thank you. Joe.